(1) Field of the Invention
This invention relates to a semiconductor device and a semiconductor device fabrication method and, more particularly, to a semiconductor device with a built-in light receiving element including the light receiving element for converting an optical signal to an electrical signal and a circuit element for processing the converted electrical signal and a method for fabricating such a semiconductor device.
(2) Description of the Related Art
Conventionally, some semiconductor devices in which a light receiving element for converting an optical signal to an electrical signal and a circuit element for processing the converted electrical signal are formed on the same substrate have been proposed (see, for example, Japanese Patent Laid-Open Publication No. 2003-110098 or Japanese Patent Laid-Open Publication No. 2003-264310). Such semiconductor devices with a built-in light receiving element are used mainly in optical pickups and the like and are formed by integrating light receiving elements, such as photodiodes, and circuit elements, such as metal oxide semiconductor (MOS) transistors or bipolar transistors, onto the same substrate by the use of an integrated circuit (IC) process.
By the way, a photodiode formed in such a semiconductor device with a built-in light receiving element must have at least a certain level of receiving sensitivity according to a use for the semiconductor device. To meet this requirement, at present an antireflection coating is usually formed in a light receiving area of a photodiode included in a semiconductor device with the built-in photodiode. This prevents input light from being reflected. This antireflection coating is made up of, for example, two layers: an oxide film and a nitride film.
However, if the antireflection coating is formed in the light receiving area of the photodiode in this way, the thickness of the antireflection coating changes in the process of forming the semiconductor device. For example, the exposed antireflection coating in a photodiode area is also etched when etching is performed for forming a wiring pattern in a circuit element area. In addition, the antireflection coating is over-etched when an interlayer dielectric or the like formed on the antireflection coating in the light receiving area of the photodiode is removed by etching to form an window from which light is inputted. As a result, the thickness of the antireflection coating may decrease.
Such a decrease in the thickness of the antireflection coating causes an increase or scatter in reflectance, resulting in a decrease in the receiving sensitivity of the photodiode. Moreover, damage caused by etching, plasma, and the like may deteriorate the leakage characteristics of the photodiode. Therefore, in order to suppress a decrease in the thickness of the antireflection coating, at present a protection layer is formed in advance on the antireflection coating by using an oxide film, polycrystalline silicon, or the like and is used for protecting the antireflection coating in the light receiving area against, for example, etching.
However, the above method in which the antireflection coating is protected by a protection layer may cause the following problems.
FIG. 25 is a schematic plan view showing an important part of a process for forming a conventional semiconductor device with a built-in divided photodiode. FIG. 26 is a schematic sectional view taken along lines D-D of FIG. 25. FIG. 27 is a schematic sectional view taken along lines E-E of FIG. 25.
In each of FIGS. 25 through 27, an important part of one process for forming a semiconductor device with a built-in photodiode including a four-divided photodiode (divided photodiode) and a MOS transistor is schematically shown. In FIG. 25, however, only an area (divided photodiode area) 100 where the divided photodiode included in the semiconductor device with a built-in photodiode is to be formed is shown.
As shown in FIGS. 26 and 27, in the divided photodiode area 100 shown in FIG. 25, an n-type epitaxial layer 102 is formed on a p-type silicon substrate 101. n-type diffusion layers (cathode area n-type diffusion layers) 103a and 103b used as cathode areas are formed in the n-type epitaxial layer 102. Each of the cathode area n-type diffusion layers 103a and 103b is defined with p-type buried diffusion layers 104a and 104b, a p-type well diffusion layer 105a, and a division section p-type diffusion layer 105b. An isolation section 106 on the periphery of the divided photodiode area 100 shown in FIG. 25 is formed by the p-type buried diffusion layer 104a and the p-type well diffusion layer 105a. A division section 107 inside the isolation section 106 shown in FIG. 25 is formed by the p-type buried diffusion layer 104b and the division section p-type diffusion layer 105b. The isolation section 106 not only defines an element area but also functions as an anode.
Excluding the isolation section 106 and the division section 107, the components in the substrate (including the p-type silicon substrate 101 and the n-type epitaxial layer 102 formed thereon) shown in FIGS. 26 and 27 are not shown in FIG. 25.
As shown in FIGS. 26 and 27, a silicon oxide film 108 is formed on the isolation section 106 and its vicinity by the method of local oxidation of silicon (LOCOS) and a second silicon oxide film 109 which is thinner than the silicon oxide film 108 is formed except on part of the cathode area n-type diffusion layer 103a and part of the cathode area n-type diffusion layer 103b. A silicon nitride film 110 is formed on the silicon oxide films 108 and 109 mainly on the division section 107 and a light receiving area 111 shown in FIGS. 25 through 27. An antireflection coating (not shown in FIG. 25) is made up of two layers of the silicon oxide film 109 and the silicon nitride film 110.
Hereinafter the “light receiving area” means not only an area in a completed photodiode to which light is inputted but an area in a photodiode under fabrication to which light is to be inputted after completion.
As shown in FIGS. 25 through 27, a polycrystalline silicon film 114 is formed on the antireflection coating in the light receiving area 111. As shown in FIGS. 26 and 27, the polycrystalline silicon film 114 is also formed in an area (cathode electrode area) 112 where a cathode electrode is to be formed and an area (anode electrode area) 113 where an anode electrode is to be formed. The polycrystalline silicon films 114 formed on the antireflection coating in the light receiving area 111, in the cathode electrode area 112, and in the anode electrode area 113 are separated from one another.
The polycrystalline silicon film 114 formed on the antireflection coating in the light receiving area 111 functions as a protection layer for protecting the antireflection coating against, for example, etching performed later. The polycrystalline silicon film 114 formed in the cathode electrode area 112 functions as part of the cathode electrode and a metal electrode is finally formed on it. Similarly, the polycrystalline silicon film 114 formed in the anode electrode area 113 functions as part of the anode electrode and a metal electrode is finally formed on it.
The divided photodiode area 100 having the above structure is formed simultaneously with the MOS transistor. If polycrystalline silicon is used as a material for a gate electrode of the MOS transistor, usually gate polycrystalline silicon is formed in an area (MOS transistor area) where the MOS transistor is to be formed simultaneously with the polycrystalline silicon films 114. Insulating films, such as a silicon oxide film and a silicon nitride film, are then formed and etched to form a sidewall. At this time the divided photodiode area 100 also suffers etching, but the antireflection coating formed in the light receiving area 111 is protected against the etching by the polycrystalline silicon film 114 formed thereon.
One of the reasons for forming the polycrystalline silicon film 114 in the divided photodiode area 100 is that the antireflection coating in the light receiving area 111 must be protected. However, the polycrystalline silicon film 114 is not formed on the antireflection coating or the like formed on, for example, an area (lead electrode area) 115 which is outside the light receiving area 111 and around the cathode electrode area 112. Accordingly, the antireflection coating or the like formed on the lead electrode area 115 is not protected against, for example, the etching.
Each of FIGS. 28 and 29 is a schematic sectional view showing an important part of an etching process. Each of FIGS. 28 and 29 is a sectional view showing the state of the semiconductor device after etching and taken along lines D-D of FIG. 25 as shown in FIG. 26.
As described above, it is assumed that the polycrystalline silicon film 114 is formed only on the antireflection coating in the light receiving area 111, in the cathode electrode area 112, and in the anode electrode area 113 and that etching is performed. In this case, the antireflection coating which is exposed in the lead electrode area 115 outside the light receiving area 111 is etched. For example, the antireflection coating on the division section 107 shown in FIG. 26 is etched. Accordingly, as shown in FIG. 28, most of the silicon nitride film 110, being the upper layer of the antireflection coating formed on the division section 107 outside the light receiving area 111, may be removed. Alternatively, as shown in FIG. 29, even the silicon oxide film 109, being the lower layer of the antireflection coating formed on the division section 107 outside the light receiving area 111, may be removed. In this case, the n-type epitaxial layer 102 and the division section p-type diffusion layer 105b get exposed.
As shown in FIG. 28, it is assumed that most of the silicon nitride film 110, being the upper layer of the antireflection coating, is removed. When treatment in an oxidation atmosphere or the chemical vapor deposition (CVD) growth of a silicon oxide film is performed later, the atmosphere or temperature has a great influence and impurities, such as boron, in the division section 107 are apt to be entrapped in the silicon oxide film 109 formed thereon. As a result, impurity concentration lowers in a surface area of the substrate and there is a strong possibility that a leakage current flows between different photodiodes.
As shown in FIG. 29, it is assumed that not only the silicon nitride film 110, being the upper layer of the antireflection coating, but also the silicon oxide film 109, being the lower layer of the antireflection coating, is removed. The n-type epitaxial layer 102 and the division section p-type diffusion layer 105b, and the cathode area n-type diffusion layers 103a and 103b which have got exposed by the etching are damaged by the etching. As a result, there appear crystal defects. In this case, there is a strong possibility that a junction leakage current flows especially in the division section 107.
The flowing of the above leakage current leads to a deterioration in the characteristics of the photodiodes and the performance of the entire semiconductor device with a built-in photodiode.
The descriptions have been given with the case where the photodiode and the MOS transistor are formed on the same substrate as an example. However, the above problems may also arise in the case where another light receiving element is formed or where a bipolar transistor is formed, depending on its structure.